1. Field of the Invention
This invention relates to a pneumatic radial tire capable of controlling separation failure at belt end.
2. Description of Related Art
There are known pneumatic tires for truck, bus and the like, which are particularly used under a heavy load and recapped plural times after wearing of a tire tread. In this type of the tire, there has recently been caused a problem that cracks are created at a side end of a belt layer in a widthwise direction thereof in the use over a long time of period and grow inward in the widthwise direction or outward in the radial direction between the belt layers with the lapse of time to cause a separation failure and hence make the use of the tire impossible. Such cracks are considered to be created due to the following fact. That is, an end portion of the tread in the pneumatic radial tire is subjected to a tensional deformation in the circumferential direction every the contacting with a flat road surface due to an influence of crown radius. At the tread end portion subjected to such a tensional deformation are located outer side end portions of plural belt layers in which cords are slantly embedded in these layers in opposite directions with respect to an equatorial plane of the tire, so that the cords crossed with each other at this position are subjected to the above tensional deformation every the arrival at the ground contacting region and repeatedly deform in a direction of crushing a diamond shape. Also, such a deformation is caused when the tread end portion of the tire rides on projections such as stones and the like during the running on bad road to largely deform rubber in this end portion. As the cords are deformed as mentioned above, the width of the belt layer becomes narrow and hence the outer end of the belt layer in the widthwise direction somewhat displaces inward in the widthwise direction repeatedly, while rubber located outward from the outer end of the belt layer in the widthwise direction is left at this position. As a result, tensile strain in the widthwise direction is caused at the boundary between the rubber and the outer end of the belt layer in the widthwise direction. Moreover, the ends of the cords embedded in the belt layer are exposed at the outer end of the belt layer in the widthwise direction and are cut faces not subjected to brass plating or the like for enhancing an adhesion to rubber. As a result the ends of the cords are repeatedly pulled apart from the surrounding rubber due to the influence of the above tensile strain and hence slight cracks are created in the rubber located in the vicinity of the cord ends exposed at the outer end of the belt layer in the widthwise direction. On the other hand, the aforementioned displacement becomes maximum at a widthwise outer end of a narrow-width belt layer arranged at an outside of a maximum-width belt layer in the radial direction and containing cords arranged in an oppositely inclination direction to cords of the maximum-width belt layer. Therefore, it is considered that the above slight cracks are first created at the widthwise outer end of the narrow-width belt layer and developed by shearing strain or the like repeatedly produced between the belt layers during running with the lapse of time to cause separation failure at belt end (hereinafter referred to as belt end separation).
In order to solve this problem, there have hitherto been proposed a method of arranging a low-hardness and thick cushion rubber between the widthwise outer end of the maximum-width belt layer and the widthwise outer end of the narrow-width belt layer for controlling the shearing strain produced between the belt layers to prevent the development of the cracks, and a method of piling a reinforcing layer containing cords extended substantially in the circumferential direction on the belt layer for controlling the increase of the size of the belt layer accompanied with the inflation under inner pressure or the running to prevent the development of the cracks.
However, these methods can develop an effect to a certain level, but have a problem that the occurrence of the belt end separation can not sufficiently be controlled.
It is, therefore, an object of the invention to provide a pneumatic radial tire capable of effectively controlling the belt end separation.
According to the invention, there is the provision of in a pneumatic radial tire comprising a radial carcass of at least one rubberized cord ply toroidally extending between a pair of bead cores, a belt superimposed about a crown portion of the carcass and comprised of at least two belt layers, and a tread arranged at an outside of the belt in a radial direction, in which the belt comprises a maximum-width belt layer containing many cords slantly arranged with respect to an equatorial plane of the tire and a narrow-width belt layer arranged at the outside of the maximum-width belt layer in the radial direction and containing many cords arranged in an oppositely inclined direction to the cords of the maximum-width belt layer, an improvement wherein a reinforcing layer containing many cords arranged in an oppositely inclined direction to the cords of the maximum-width belt layer is arranged so as to overlap with an outer end portion of the maximum-width belt layer located outward from an outer end of the narrow-width belt layer in the widthwise direction thereof.
When the cords embedded in the widthwise outer end portions of the maximum-width belt layer and the narrow-width belt layer arrive at a ground contact region during the running of the pneumatic radial tire, a diamond shape defined by these cords is deformed in a crushed direction due to the influence of a crown curvature, whereby the widthwise outer end of the narrow-width belt layer displaces inward in the widthwise direction at maximum. Therefore, when the reinforcing layer is arranged to overlap with the widthwise outer end portion of the maximum-width belt layer located outward from the widthwise outer end of the narrow-width belt layer in the widthwise direction and contains many cords arranged in an oppositely inclination direction to the cords of the maximum-width belt layer, a diamond shape defined by the cords embedded in the reinforcing layer and the cords embedded in the maximum-width belt layer are also deformed in a crushed direction due to the same tensional deformation in the circumferential direction as mentioned above, so that the reinforcing layer, widthwise outer end portion of the maximum-width belt layer and rubber surrounding them (including rubber located outward from the widthwise outer end of the narrow-width belt layer in the widthwise direction) displace inward in the widthwise direction together, whereby the tensile strain in the widthwise direction produced at the boundary between the widthwise outer end of the narrow-width belt layer and the rubber located outward from the widthwise outer end in the widthwise direction is decreased (or it is a compression strain according to circumstances). As a result, the occurrence and development of cracks in the rubber located in the vicinity of cord ends of the narrow-width belt layer are effectively controlled and hence the belt end separation is effectively prevented.
In a preferable embodiment of the invention, the cords of the reinforcing layer are inextensible cords and the reinforcing layer is closely arranged at the inside of the maximum-width belt layer in the radial direction, or the widthwise outer end of the reinforcing layer is located inward from the widthwise outer end of the maximum-width belt layer in the widthwise direction, whereby the belt end separation of the narrow-width belt layer can surely be prevented while controlling the separation failure at the widthwise outer end of the reinforcing layer.
In another preferable embodiment of the invention, the widthwise inner end of the reinforcing layer is located in the vicinity of the widthwise outer end of the narrow-width belt layer and an inclination angle A of the cord embedded in the reinforcing layer with respect to the equatorial plane of the tire is within a range of 40xc2x0 to 60xc2x0. In this case, the belt end separation of the narrow-width belt layer can further be prevented.
In the other preferable embodiment of the invention, the reinforcing layer is extended inward in the widthwise direction to form a one-piece body on the equatorial plane and an inclination angle B of the cord embedded in the reinforcing layer is not less than 34xc2x0 with respect to the equatorial plane. In this case, the hoop effect of the belt can be enhanced while simplifying the production.
In a further preferable embodiment of the invention, the cords of the reinforcing layer are organic fiber cords and the reinforcing layer is arranged at the outside of the maximum-width belt layer in the radial direction. Even in this case, the belt end separation of the narrow-width belt layer can effectively be prevented.
In a still further preferable embodiment of the invention, the widthwise inner end of the reinforcing layer is located at the outside of the narrow-width belt layer in the radial direction and inward from the widthwise outer end thereof in the widthwise direction, and the widthwise outer end of the reinforcing layer is located outward from the widthwise outer end of the maximum-width belt layer in the widthwise direction, or further an inclination angle C of the cord embedded in the reinforcing layer is not less than 7xc2x0 with respect to the equatorial plane. In this case, the belt end separation of the narrow-width belt layer can surely be prevented.